Temperature-dependent Raman spectra and thermal expansion of MgP2O6.

The Raman spectra and thermal expansion of MgP2O6 metaphosphate were investigated at various temperatures up to 1073 K at ambient pressure. No temperature-induced phase transition was observed in this study. The effect of temperature on the Raman active vibrations and unit cell parameters was determined. All the observed Raman active bands of MgP2O6 showed linear temperature dependences in the range of -2.61 × 10-2 âˆ¼ -0.49 × 10-2 cm-1 K-1. The thermal expansion coefficient of MgP2O6 was estimated to be 3.21(2) × 10-5 K-1. An axial anisotropic thermal expansion exists and the c-axis shows the smallest thermal expansion. The isobaric mode Grüneisen parameters of MgP2O6 were calculated. The obtained results were compared with other compounds in the MgO-P2O5 system.

Raman spectroscopic and X-ray diffraction study of α- and ß-Mg2P2O7 at various temperatures.

Raman spectra and X-ray diffraction patterns of Mg2P2O7 polymorphs (α- and ß-phase) were investigated at various temperatures up to 1073 K at ambient pressure. Typical Raman spectra and X-ray diffraction patterns were observed for the reversible phase transition between low-temperature α-Mg2P2O7 and high- temperature ß-Mg2P2O7 during heating and cooling. The effect of temperature on the Raman vibrations for the two Mg2P2O7 polymorphs was quantitatively analyzed. All the observed Raman active bands of the two Mg2P2O7 polymorphs showed linear temperature dependence with different slopes. The quantitative temperature dependences of the Raman bands are -4.01 × 10-2 âˆ¼ 1.94 × 10-2 and -2.31 × 10-2 âˆ¼ -0.44 × 10-2 cm-1 K-1 for α- and ß-Mg2P2O7, respectively. The force constant evolution of [P2O7]4- stretching vibrations and the temperature derivatives for both α- and ß-Mg2P2O7 were also determined. The thermal expansion coefficient of ß-Mg2P2O7 was estimated at 2.97(8) × 10-5 K-1. Hence the isobaric mode Grüneisen parameters of ß-Mg2P2O7 were calculated.

Phase transition of Mg3(PO4)2 polymorphs at high-temperature: In-situ synchrotron X-ray diffraction and Raman spectroscopic study.

In-situ synchrotron X-ray diffraction and Raman spectroscopy were used to study the stabilities, thermal expansion and vibrational modes of synthetic Mg3(PO4)2. The polymorphs (Mg3(PO4)2-I, II, III) were investigated in the temperature range of 299 âˆ¼ 1173 K at ambient pressure. An irreversible phase transition was observed for both Mg3(PO4)2-II and Mg3(PO4)2-III, whereas Mg3(PO4)2-I is stable in the present study. Based on the in-situ synchrotron X-ray diffraction and Raman spectroscopic measurements, Mg3(PO4)2-II and Mg3(PO4)2-III transform to Mg3(PO4)2-I at 1073 K and 1023 K, respectively. The volumetric thermal expansion coefficients of Mg3(PO4)2-I, II and III were determined as 3.31(4) × 10-5 K-1, 3.91(4) × 10-5 K-1, and 3.25(5) × 10-5 K-1, respectively. All three Mg3(PO4)2 polymorphs show axial thermal expansive anisotropy since the thermal expansion coefficients along different axes are inconsistent. The effect of temperature on the Raman vibrations of the three Mg3(PO4)2 polymorphs was quantitatively analyzed. And the isobaric mode Grüneisen parameters of three Mg3(PO4)2 polymorphs are calculated, which are in the range of 0.07 âˆ¼ 3.54.